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The process begins when the body's immune system detects the invading virus and brings out the equivalent of heavy artillery. The vascular system goes on full alert to seek and destroy the enemy.

Picture using a shotgun to kill a mosquito flying indoors, with blast after blast destroying all the doors and windows, the walls and floors, the ceilings too, as the insect escapes. When it is all over, there is more damage done to the body than to the virus.

"People basically bleed out," Jecha said.

Some idea of what could happen comes from the 1918-19 flu pandemic, which was caused by a strain related to H5N1.

According to the Centers for Disease Control and Prevention, many people died within a few days during the 1918-19 pandemic. Nearly half of the victims were young, healthy adults.

Newspapers from 1919 reported that people who had no symptoms were suddenly struck ill with symptoms that included a cough, high fever and extreme fatigue. Within hours, they were too feeble to walk, then dead the next day.

Victims coughed up blood as blood vessels became porous and fluids filled the lungs. There was uncontrollable hemorrhaging in the lungs and they drowned in their own body fluids, Jecha said.

The virus, as in the cases of H5N1 that have been seen, caused victims' immune systems to go all-out to destroy the virus. In young adults, their strong immune systems nuked their bodies.

But what's perhaps most frightening is that the 1918-19 pandemic had a kill rate of less than 1 percent, said Jecha. Known cases of H5N1 in people have had a kill rate exceeding 50 percent.

Which is why health officials are so concerned about the potential for the H5N1 virus to mutate into a form that can be transmitted between humans.

There have been three pandemic flu outbreaks in the 20th century. The first was the deadly 1918-19 Spanish flu. The Asian flu came in 1957-58, and the Hong Kong flu hit in 1968-69. According to the CDC, the Hong Kong flu virus still is circulating.

The CDC notes that both the 1957-58 and 1968-69 pandemics were caused by viruses containing a combination of genes from a human influenza virus and an avian flu virus. The 1918-19 pandemic virus appears to have had an avian origin.

For more information about the avian flu and how it could become a pandemic flu, go to www.cdc.gov/flu/avian/index.htm.